Of the numerous classes of curing agents for epoxy resins, amines and amine derivatives offer the greatest utility for curing of epoxy resins. As a group, these are the only available materials that offer adequate potential for curing, both in thin films and mass, at room temperature. Aromatic amine curatives have been used extensively. However, due to toxicity characteristics, the use of aromatic amines has substantially curtailed. Current amine-based curing agents are primarily either aliphatic or cycloaliphatic in nature such as diethylene triamine, isophorone diamine and 1,2-diaminocyclohexane.
Many commercial curing agent formulations are based on these aliphatic and cycloaliphatic amines which have been modified in some way, either to refine performance aspects, improve combining ratios with epoxy resins or decrease toxicity of the base amine. The most used commercially available amines are fairly small molecules with relatively large amounts of nitrogen. Low molecular weight members are sufficiently volatile that hazardous concentrations can accumulate within confined air spaces.
As organic bases, amines react readily with atmospheric moisture and carbon dioxide to form the corresponding amine carbonate. This serves to "block" or inhibit reaction of amine with epoxide. This phenomenon is most readily evident in curing of thin films. Applied coatings of epoxy resin blended with unmodified amine will remain largely uncured unless baked. Also, amine carbonate is insoluble in the matrix, and will remain on the surface of the coating as a hazy film, referred to as "bloom," "blush" or "sweat-out."
Amines are typically combined with epoxy resins such that a ratio of one amine hydrogen unit per epoxide group is employed. The lower molecular weight ethyleneamines have the capability to form tightly cross linked networks when used as epoxy curing agents. However, at ambient temperature, curing rate slows as polymer viscosity increases, so that curing is incomplete unless accelerators are used or temperature is increased to compensate. In order to be useful for ambient temperature curing purposes, low molecular weight, highly functional amines must be modified in some way to minimize carbonation and improve curing performance and reduce volatility.
Mannich Bases are examples of amine modification which minimizes carbonation. Mannich Base compounds are products based on the reaction of an aldehyde, generally formaldehyde, a phenolic compound and an amine. Commercially available Mannich Bases are based on formaldehyde, either utilized as an aqueous solution of the gas, or in polymerized form as paraformaldehyde.
High levels of aldehyde are beneficial in providing a Mannich Base having in combination with liquid epoxy resins (1) rapid cure, both at ambient and reduced temperature, (2) good mechanical strength and hardness properties, and (3) generally good chemical resistance characteristics. However, such high levels tend to advance product viscosity beyond a useable range. Low viscosity is desirable for most applications involving this class of curing agent.